When I set up on staging, I used CiviCRM Version 5.13.4, and the various Profile options were available, including :
Used For Standalone Form or Directory Search Views [ x ] User Registration [ x ] User Account [ x ] soft_credit [ x ] CiviContribute [ x ] on_behalf

However, now that I'm setting up on live with Version 5.13.5, my only available options are:
Used For Standalone Form or Directory Search Views [ x ] User Registration [ x ] User Account

Okay, so that explains a lot of things that I was trying to understand about profiles but couldn't find mentioned in the documentation. Since I was working on a live site, I didn't want to mess anything up and was proceeding with caution. But this was very helpful, and I will forge ahead!
– Titania PorterJul 25 '19 at 18:22